The phenomenal growth of female headship in the U.S. over the past twenty years has been widely discussed and investigated by the research community. The research proposed here builds upon prior research by conducting formal tests of four different hypotheses that have been suggested for the determinants of female headship. The theories to be tested posit that female headship is heavily influenced by (1) labor market opportunities for women, most usually proxied by the female wage rate; (2) the level of AFDC and other transfer benefits; (3) labor market opportunities for men, especially unskilled men, as proxied by the wage rate; and (4) changes in the sex ratio in low-income populations that have lowered the number of men available on the marriage market. Two different criteria will be used to judge the importance of each of the four: their statistical significance and explanatory power in microdata analyses, and their ability to explain the time series increase in female headship. A large amount of data from different sources will be assembled and analyzed, including the CPS, PSID, and SIPP. Models appropriate for cross sections, repeated cross sections, and panel data will be estimated as relevant for each type of data.